1. Field of the Invention
The present invention relates to a magnetic recording medium, and particularly to a coating-type magnetic recording medium which is suitable for the high density recording.
2. Description of the Prior Art
In recent years, various methods have been proposed for satisfying the demand for the high density recording of the magnetic recording medium, for example, using further fined ferromagnetic powder, using alloy magnetic powder, increasing magnetization and coercive force of the magnetic layer, smoothing a surface of a magnetic layer of the medium, achieving vertical (or oblique) orientation of the magnetic powder in the magnetic layer, reducing the magnetic layer in thickness, or the like. In addition to the demand for the high density recording, it has been also required to reduce size of the magnetic recording medium. Accordingly, it has been the big aim to reduce a thickness of the medium, particularly, a thickness of the magnetic layer. If, however, the magnetic layer is simply reduced in thickness, the effect of a final calendering treatment is not notably reflected. Thus, a surface roughness of the magnetic layer is not improved to result in a low electromagnetic transducer property.
To cope with such a problem, a method has been proposed to provide an undercoating layer between a magnetic layer and a flexible substrate for smoothing a surface of the medium, particularly, a surface of the magnetic layer.
For example, Japanese First (unexamined) Patent Publication No. SHO 57-198536 has proposed a method of forming a flexible substrate, applying an undercoating layer on the flexible substrate, and forming a magnetic layer thereon after drying the undercoating layer. Further, Japanese Second (examined) Patent Publication No. HEI 5-59490 has proposed a method of applying an undercoating layer on a flexible substrate and forming a magnetic layer while the undercoating layer is still in a wet state. Although it is not for exclusive use in the magnetic recording medium, Japanese First (unexamined) Patent Publication No. SHO 58-29626 has proposed a method for producing a polyester film for improving an adhesive property, wherein a coating layer (adhesive layer) containing polyester as a main component is applied on a polyester film base, and then a stretching (or orientation) treatment and heat treatment are performed to achieve orientation and crystallization thereof.
However, in the method proposed in Japanese First Patent Publication SHO 57-198536, although the functions of the undercoating layer and the magnetic layer can be optimized, a fine thickness variation is liable to occur upon the formation of the undercoating layer, thereby leading to a complex process control. Although it is possible to perform a calendering treatment after drying the undercoating layer for preventing such a fine thickness variation, this may cause the production processes to be further complicated so as to lower the production efficiency. Further, since the magnetic layer is formed after drying the undercoating layer, the magnetic layer is susceptible to lowering of the orientation.
On the other hand, in the method proposed in Japanese Second Patent Publication No. HEI 5-59490, it is possible to provide the undercoating layer and the magnetic layer through one coating process so that the production processes can be simplified as compared with the foregoing method. However, it is possible that disturbance of the interface between the undercoating layer and the magnetic layer and mixing of the coating materials are caused to incur output fluctuation of the produced magnetic recording medium. Further, since the application of the undercoating layer and the magnetic layer can not be performed without considering matching of the coating materials of the undercoating and magnetic layers, selecting ranges of the coating materials are so limited that compositions themselves of the undercoating and magnetic layers can not be optimized.
On the other hand, the method disclosed in Japanese First Patent Publication No. SHO 58-29626 only aims at the adhesive property of the film relative to one to be attached thereto. Thus, setting of a specification of the base film as an undercoating layer in the magnetic recording medium having a thin magnetic layer and suitable for the high density recording is not proposed in this publication.
As a technique similar to the foregoing polyester film producing method, Japanese First (unexamined) Patent Publication No. HEI 4-248116 has proposed a magnetic recording medium, wherein a coating liquid mainly containing carbon black and water-soluble polyurethane resin is applied on a polyester film and then a magnetic layer is formed after a stretching (or orientation) treatment. However, this proposed technique mainly aims at reducing an electrical resistance, wherein the magnetic layer has a large thickness of 4 .mu.m and the carbon black has a large average particle size in the range of 0.1 to 0.5 .mu.m, and thus, does not aim at improving a surface roughness of an applied thin magnetic layer.
Further, since the carbon black has poor dispersibility, a surface of the base film is liable to be roughened due to agglomeration. Thus, the carbon black is not suitable for the magnetic recording medium aiming at the high density recording. Further, since the carbon black is impurities to be removed upon recycling the base film, it is not preferable also in view of the effective use of resources as a social request.
The present invention has been made under these circumstances and has an object to provide a magnetic recording medium which is capable of achieving high outputs in the region of shorter wavelength and excellent in running durability and productivity.